Bad Transmission?

I just got a free 1991 honda civic dx auto that wasnt running yesterday. I towed it home and jumped the battery and now the motor runs absolutely perfectly, but the transmission occasionaly makes a loud grinding nois and it slips to where the motor free revs. someone was telling me i just need trans fluid but to me it sounds like something is hella messed up. Now i now nothing about auto transmissions, so i just thought i would ask you guys before i buy another transmission.
Thanks

Also i havnt been able to find anything out about doing a 5speed swap (i searched), but if i could get any info on that i would definently appreciate that also.

Check the fluid level and if its low add some, use Honda ATF. You may also just try a fluid change. From the slipping it sounds like it could be a bad torque converter. A manual swap isn't that hard. You need a manual tranny, pedal assembly, clutch cable, manual ecu and of course a clutch/ flywheel.